A new optical atomic clock’s heart is as small as a coffee bean

Portable atomic clocks are on their way to an upgrade.

Today’s small, battery-operated atomic clocks track time by counting oscillations of light absorbed by cesium atoms (SN: 9/4/04, p. 50). That light oscillates billions of times per second. Now, a miniature version of a type of atomic clock called an optical clock uses light tuned to rubidium atoms, and beats trillions of times per second (SN Online: 5/20/19). Dividing time into such short intervals allows this atomic timepiece to keep time much more reliably than other clocks, researchers report May 20 in Optica. 

Ordinarily, the chamber of atoms at the heart of an optical clock might be a meter across. The new mini optical clock uses an atom chamber a mere 3 millimeters across mounted on a silicon chip. “I was very surprised they were able to make an optical clock this size,” says Silvio Koller, who worked on optical clocks at the National Metrology Institute of Germany in Braunschweig.

A new generation of small-scale optical clocks could better coordinate the flow of data through telecommunication networks, or sync up far-flung telescopes to make astronomical observations (SN: 4/27/19, p. 7).

The “pendulum” inside the new optical clock is a laser tuned to about 385.285 terahertz — that is, its light undulates 385.285 trillion times per second. To ensure that the laser’s oscillations don’t fall out of rhythm, half of the beam feeds into the tiny chamber of rubidium atoms, which absorb light at precisely this frequency. Monitoring whether the rubidium atoms are absorbing light tells the laser whether it needs to dial its frequency slightly up or down to keep time more precisely.

Modern electronics can’t actually count the individual 385-terahertz ticks of this laser because they’re too fast, says study coauthor Zachary Newman, a physicist at the National Institute of Standards and Technology in Boulder, Colo.

So the optical clock uses two components called frequency combs, also mounted on tiny chips, to translate the laser’s rapid-fire beats into slower, countable ticks. This works similar to the way a set of gears can translate the rapid spin of a small disk into the slower rotation of a larger disk (SN: 10/22/11, p. 22).

The optical clock ultimately produced ticks paced at 22 gigahertz — about twice as fast as those of cesium-based metronomes. But because the optical clock’s gigahertz ticks are based on the much shorter, terahertz beats, they’re far more precise than the gigahertz ticks of cesium clocks. The duration of each second counted out by the chip-scale optical clock matched every other, down to about five trillionths of a second.

That’s roughly 50 times better than the current cesium-based chip-scale clocks, says study coauthor Matthew Hummon, also a physicist at NIST in Boulder.

Even though the new optical clock is pint-size compared with its predecessors, it isn’t a pocket watch yet. The chip-scale atom chamber and frequency combs are hooked up to supporting electronics that fill two tables. “Eventually we’d like to get this technology to be truly handheld and battery powered,” Hummon says.